Photovoltaic device including flexible substrate or inflexible substrate and method for manufacturing the same

ABSTRACT

Disclosed is a photovoltaic device. The photovoltaic device includes: a substrate; a first electrode placed on the substrate; a second electrode which is placed opposite to the first electrode and which light is incident on; a first unit cell being placed between the first electrode and the second electrode, and including an intrinsic semiconductor layer including crystalline silicon grains making the surface of the intrinsic semiconductor layer toward the second electrode textured; and a second unit cell placed between the first unit cell and the second electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2010-0027280 filed on Mar. 26, 2010, the entirety ofwhich is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a photovoltaic device including aflexible substrate or an inflexible substrate and a method formanufacturing the same.

BACKGROUND OF THE INVENTION

Recently, because of a high oil price and a global warming phenomenonbased on a large amount CO₂ emission, energy is becoming the mostimportant issue in determining the future life of mankind. Even thoughmany renewable energy technologies including wind force, bio-fuels, ahydrogen/fuel cell and the like have been developed, a photovoltaicdevice using sunlight is in the spotlight in that solar energy, theorigin of all energies, is an almost infinite clean energy source.

The sunlight incident on the surface of the earth has an electric powerof 120,000 TW. Thus, theoretically, if a photovoltaic device having aphotoelectric conversion efficiency of 10% covers only 0.16% of the landsurface of the earth, it is possible to generate electric power of 20 TWthat is twice as much as the amount of energy globally consumed duringone year.

Actually, the world photovoltaic market has explosively grown by almost40% of an annual growth rate for the last ten years. Now, a bulk-typesilicon photovoltaic device occupies a 90% of the photovoltaic devicemarket share. The bulk-type silicon photovoltaic device includes asingle-crystalline silicon photovoltaic device and a multi-crystallineor a poly-crystalline silicon photovoltaic device and the like. However,productivity of a solar-grade silicon wafer which is the main materialof the photovoltaic device is not able to fill the explosive demandthereof, so the solar-grade silicon wafer is globally in short supply.Therefore, this shortage of the solar-grade silicon wafer is a hugethreatening factor in reducing the manufacturing cost of a photovoltaicdevice.

Contrary to this, a thin-film silicon photovoltaic device based on ahydrogenated amorphous silicon (a-Si:H) allows to reduce a thickness ofa silicon layer equal to or less than 1/100 as large as that of asilicon wafer of the bulk-type silicon photovoltaic device. Also, itmakes possible to manufacture a large area photovoltaic device at alower cost.

Meanwhile, a single-junction thin-film silicon photovoltaic device islimited in its achievable performance. Accordingly, a double junctionthin-film silicon photovoltaic device or triple junction thin-filmsilicon photovoltaic device having a plurality of stacked unit cells hasbeen developed, pursuing high stabilized efficiency.

The double junction or triple junction thin-film silicon photovoltaicdevice is referred to as a tandem-type photovoltaic device. The opencircuit voltage of the tandem-type photovoltaic device corresponds to asum of each unit cell's open circuit voltage. Short circuit current isdetermined by a minimum value among the short circuit currents of theunit cells.

Regarding the double junction thin-film silicon photovoltaic device orthe triple junction thin-film silicon photovoltaic device, research isbeing devoted to an electrode surface texture in order to improveefficiency by means of light scattering.

SUMMARY OF THE INVENTION

One aspect of the present invention is a photovoltaic device. Thephotovoltaic device includes: a substrate; a first electrode placed onthe substrate; a second electrode which is placed opposite to the firstelectrode and which light is incident on; a first unit cell being placedbetween the first electrode and the second electrode, and including anintrinsic semiconductor layer including crystalline silicon grainsmaking the surface of the intrinsic semiconductor layer toward thesecond electrode textured; and a second unit cell placed between thefirst unit cell and the second electrode.

Another aspect of the present invention is a method for manufacturingthe photovoltaic device of the present invention. The method includes:forming an n-type semiconductor layer of the first unit cell; forming anintrinsic semiconductor layer of the first unit cell on the n-typesemiconductor layer, the intrinsic semiconductor layer including anamorphous silicon based material surrounding crystalline silicon grain;etching the surface of the intrinsic semiconductor layer; and forming ap-type semiconductor layer of the first unit cell on the intrinsicsemiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b show a photovoltaic device according to an embodimentof the present invention.

FIGS. 2 a to 2 h show a method for manufacturing the photovoltaic deviceaccording to the embodiment of the present invention.

DETAILED DESCRIPTION

Hereinafter, a photovoltaic device according to an embodiment of thepresent invention will be described in detail with reference to theaccompanying drawings.

FIG. 1 shows a photovoltaic device according to an embodiment of thepresent invention. As shown in FIG. 1, the photovoltaic device accordingto the embodiment of the present invention includes a substrate 100, afirst electrode 110, a second electrode 120, a first unit cell 130 and asecond unit cell 140.

The substrate 100 is a flexible substrate or an inflexible substrate.The flexible substrate includes either a metal substrate made ofstainless steel or aluminum foil, or a plastic substrate made ofPolyethylene Naphthalate (PEN) or Poly Ethylene Terephthalate (PET). Inthe metal substrate, an insulating film is formed on the surface of themetal substrate in order to insulate the metal substrate from anelectrode formed on the metal substrate. The inflexible substrate mayinclude a glass substrate.

The first electrode 110 is placed on the substrate 100. In an n-i-p typephotovoltaic device, since light is incident through the second unitcell 140, the first electrode 110 adjacent to the substrate 100 may ormay not have light transmittance. Therefore, the first electrode 110 maybe formed of a metallic material or be formed of a light-transmittingconductive material like transparent conductive oxide (TCO).

The second electrode 120 is placed opposite to the first electrode 110.Light is incident on the second electrode 120. As described above, inthe n-i-p type photovoltaic device, since light is incident through thesecond unit cell 140, light is incident on the second electrode 120prior to the first electrode 110. Therefore, the second electrode 120may be formed of a light transmitting conductive material.

The first unit cell 130 is located between the first electrode 110 andthe second electrode 120, and includes an intrinsic semiconductor layer133 including crystalline silicon grains forming a textured surfacetoward the second electrode 120. The first unit cell 130 includes ann-type semiconductor layer 131, an intrinsic semiconductor layer 133 anda p-type semiconductor layer 135, which are sequentially stacked.Therefore, among the n-type semiconductor layer 131, the intrinsicsemiconductor layer 133 and the p-type semiconductor layer 135, then-type semiconductor layer 131 is the closest to the first electrode110.

In the embodiment of the present invention, since light is incident onthe first unit cell 130 posterior to the second unit cell 140, the firstunit cell 130 absorbs more light of a long wavelength than light of ashort wavelength. Therefore, in order to easily absorb the light of along wavelength, the intrinsic semiconductor layer 133 of the first unitcell 130 includes a hydrogenated microcrystalline silicon based materiallayer. The hydrogenated microcrystalline silicon based material layer iscomposed of hydrogenated microcrystalline silicon (i-μc-Si:H) orhydrogenated microcrystalline silicon germanium (i-μc-SiGe:H).

The hydrogenated microcrystalline silicon based material layer includesa hydrogenated amorphous silicon based material 133 a and crystallinesilicon grains 133 b surrounded by the hydrogenated amorphous siliconbased material 133 a. The crystalline silicon grains 133 b projectstoward the second electrode 120, so that the surface toward the secondelectrode 120 among the surfaces of the intrinsic semiconductor layer133 is textured. A method for projecting the crystalline silicon grains133 b toward the second electrode 120 will be described later in detail.

As such, due to the textured surface formed by the crystalline silicongrains 133, the surfaces of an n-type semiconductor layer 141, anintrinsic semiconductor layer 143 and a p-type semiconductor layer 145of the second unit cell 140 are also textured. Hereby, light incident onthe second unit cell 140 may be scattered.

In the n-i-p type photovoltaic device, though the surface of either thesubstrate 100 or the first electrode 110 is textured, the surface of thesecond unit cell 140 on which light is incident is not textured enoughsince the thicknesses of the first unit cell 130 and the second unitcell 140 are large. That is, since the thicknesses of the first unitcell 130 and the second unit cell 140 are large, the surfaces of thefirst unit cell 130 and the second unit cell 140 can be flattened whilethe first unit cell 130 and the second unit cell 140 are formed bydeposition.

On the contrary, in the photovoltaic device according to the embodimentof the present invention, the crystalline silicon grains of the firstunit cell 130 project toward the second unit cell 140. Therefore, thesurface of the second unit cell 140 is textured when the second unitcell 140 is formed on the first unit cell 130.

As described above, an average crystal volume fraction of the intrinsicsemiconductor layer 133 including the hydrogenated microcrystallinesilicon based material layer is equal to or more than 25% and equal toor less than 75%.

When the average crystal volume fraction of the intrinsic semiconductorlayer 133 is equal to or more than 25%, an incubation layer includinghydrogenated amorphous silicon can be prevented from being formed eitherbetween the intrinsic semiconductor layer 133 and the p-typesemiconductor layer 135 or between the intrinsic semiconductor layer 133and the n-type semiconductor layer 131. Accordingly, hole transition orelectron transition is easily performed, so that electron-holerecombination is reduced and photoelectric conversion efficiency isimproved.

When the average crystal volume fraction of the intrinsic semiconductorlayer 133 is equal to or less than 75%, the size of the crystallinesilicon grain is prevented from excessively increasing and the volume ofgrain boundary is hereby prevented from increasing.

The second unit cell 140 is located between the first unit cell 130 andthe second electrode 120. The second unit cell 140 includes an n-typesemiconductor layer 141, an intrinsic semiconductor layer 143 and ap-type semiconductor layer 145, which are sequentially stacked.Therefore, among the n-type semiconductor layer 141, the intrinsicsemiconductor layer 143 and the p-type semiconductor layer 145, thep-type semiconductor layer 145 is the closest to the second electrode120.

Here, the p-type semiconductor layer 135 of the first unit cell 130 andthe p-type semiconductor layer 145 of the second unit cell 140 are dopedwith group III impurities. The n-type semiconductor layer 131 of thefirst unit cell 130 and the n-type semiconductor layer 141 of the secondunit cell 140 are doped with group V impurities.

Meanwhile, as shown in FIG. 1 b, the photovoltaic device according tothe embodiment of the present invention further includes an intermediatereflector 150 located between the first unit cell 130 and the secondunit cell 140. An average refractive index of the intermediate reflector150 is equal to or more than 1.7 and is equal to or less than 2.5 at awavelength of 600 nm. In such a state, refractive index matching occurs.As a result, light of a short wavelength is reflected to the second unitcell 140, so that the photoelectric conversion efficiency of thephotovoltaic device is improved.

Next, a method for manufacturing the photovoltaic device according tothe embodiment of the present invention will be described in detail withreference to the drawings.

As shown in FIG. 2 a, provided is a substrate 100 on which the firstelectrode 110 is formed by a sputtering method. Before the firstelectrode 110 is formed, a cleaning process may be performed on thesubstrate 100. When the first electrode 110 is made of a metallicmaterial, a ZnO layer may be formed between the first electrode 110 andthe substrate 100 in order to increase an adhesive strength between thefirst electrode 110 and the substrate 100. Since the substrate 100 hasbeen described above in detail, the description thereof will be omitted.

As shown in FIG. 2 b, the n-type semiconductor layer 131 of the firstunit cell 130 is formed on the first electrode 110.

As shown in FIG. 2 c, the intrinsic semiconductor layer 133 of the firstunit cell 130 is formed on the n-type semiconductor layer 131. Here, theintrinsic semiconductor layer 133 of the first unit cell 130 includesthe amorphous silicon based material 133 a surrounding the crystallinesilicon grains 133 b. That is, the intrinsic semiconductor layer 133 ofthe first unit cell 130 can include a hydrogenated microcrystallinesilicon based material layer having a mixed phase. As described above,the average crystal volume fraction of the intrinsic semiconductor layer133 is equal to or more than 25% and is equal to or less than 75%.

As shown in FIG. 2 d, the surface of the intrinsic semiconductor layer133 is etched. Here, the surface of the intrinsic semiconductor layer133 is etched by using a dry-etching process such as a hydrogen plasmaetching process or an argon plasma etching process. When thephotovoltaic device according to the embodiment of the present inventionincludes a flexible substrate, a chemical etching process transforms ordamages the flexible substrate composed of a metallic material or apolymer. However, the dry-etching process has less influence on theflexible substrate than the chemical etching process.

Accordingly, the intrinsic semiconductor layer 133 is selectivelyetched. In other words, as described above, the intrinsic semiconductorlayer 133 includes the crystalline silicon grains and the amorphoussilicon based material, which have different etch rates from each other.The amorphous silicon based material is etched more quickly than thecrystalline silicon grain under the same condition. Therefore, throughthe dry-etching process, the crystalline silicon grains surrounded bythe amorphous silicon based material are projected. The surface of theintrinsic semiconductor layer 133 is textured due to the crystallinesilicon grains projected by the aforementioned method.

Further, hole transport within the intrinsic semiconductor layer 133 isenhanced by removing the amorphous silicon based material. Accordingly,an open circuit voltage of the photovoltaic device and the photoelectricconversion efficiency of the photovoltaic device can be improved.

As shown in FIG. 2 e, a passivation layer (PL) having a thickness ofabout 5 nm is formed on the etched intrinsic semiconductor layer 133.When the surface of the intrinsic semiconductor layer 133 is etched,electron-hole recombination can be increased on the surface of theintrinsic semiconductor layer 133. Therefore, the passivation layer (PL)is formed on the etched surface of the intrinsic semiconductor layer133. Here, the passivation layer (PL) consists of the hydrogenatedmicrocrystalline silicon based material. Since the intrinsicsemiconductor layer 133 also includes the hydrogenated microcrystallinesilicon based material, the manufacturing process thereof can besimplified.

As shown in FIG. 2 f, the p-type semiconductor layer 135 of the firstunit cell 130 is formed on the intrinsic semiconductor layer 133. Here,the p-type semiconductor layer 135 is composed of hydrogenatedmicrocrystalline silicon (p-μc-Si:H).

As shown in FIG. 2 g, the n-type semiconductor layer 141, the intrinsicsemiconductor layer 143 and the p-type semiconductor layer 145 aresequentially stacked on the first unit cell 130. Since the second unitcell 140 is formed on the first unit cell 130 having the texturedsurface, the surface of the second unit cell 140 is also textured. Theintrinsic semiconductor layer 143 of the second unit cell 140 may bemade of hydrogenated amorphous silicon (i-a-Si:H), hydrogenatedamorphous silicon carbide (i-a-SiC:H), hydrogenated amorphous siliconoxide (i-a-SiO:H), hydrogenated proto-crystalline silicon (i-pc-Si:H) orhydrogenated proto-crystalline silicon having a multi-layer structure.

As shown in FIG. 2 h, the second electrode 120 is formed on the secondunit cell 140 by means of a sputtering method or an LPCVD method. Here,the second electrode 120 may be made of indium tin oxide (ITO), SnO₂:For ZnO.

As such, the surface of the second electrode 120 is textured by thecrystalline silicon grains 133 b. The textured surface of the secondelectrode 120 has an average pitch equal to or more than 50 nm and equalto or less than 500 nm in order to scatter incident light. As shown inFIG. 2 h, the average pitch corresponds to an average of distances “P”between two adjacent convex-up portions of the textured surface. Theaverage pitch of the second electrode on which light is incident isequal to or more than 50 nm and equal to or less than 500 nm, thetextured surface of the second electrode 120 is able to sufficientlyscatter visible light.

Meanwhile, since the surface of the second electrode 120 is textured bythe crystalline silicon grains 133 b, it is not necessary to form theintermediate reflector 150 with a large thickness. For example, for thepurpose that the surface of the second electrode 120 is textured bydepositing the intermediate reflector 150 including ZnO, the surface ofthe intermediate reflector 150 should be fully textured. In order thatthe surface of the intermediate reflector 150 is textured as mentionedabove, the thickness of the intermediate reflector 150 is required to beat least 1.6 μm. The processing time thereof is hereby increased.

Particularly, when the intermediate reflector 150 is formed in aroll-to-roll type manufacturing equipment, the substrate 100 iscontinuously moving, so that a time during which the substrate 100 staysin a process chamber is constant. Therefore when the thickness andprocessing time of the intermediate reflector 150 are increased, thenumber of the process chambers for forming the intermediate reflector150 may increase.

Meanwhile, in the photovoltaic device according to the embodiment of thepresent invention, since the surface of the second electrode 120 istextured enough by the crystalline silicon grains 133 b, the thicknessof the intermediate reflector 150 can be prevented from being increased.The thickness of the intermediate reflector 150 according to theembodiment of the present invention is equal to or more than 20 nm andequal to or less than 200 nm. When the thickness of the intermediatereflector 150 is less than 20 nm, the refractive index matching is hardto obtain. When the thickness of the intermediate reflector 150 islarger than 200 nm, the amount of light absorbed by the intermediatereflector 150 is increased according to the increase of the thickness ofthe intermediate reflector 150. As a result, even though thephotovoltaic device according to the embodiment of the present inventionincludes the flexible substrate 100, it is possible to prevent theprocessing time from being increased.

In the embodiment of the present invention, though the double-junctiontandem photovoltaic device has been described, the present invention isapplicable to an intermediate cell or a bottom cell of a triple-junctiontandem photovoltaic device.

While the embodiment of the present invention has been described withreference to the accompanying drawings, it can be understood by thoseskilled in the art that the present invention can be embodied in otherspecific forms without departing from its spirit or essentialcharacteristics. Therefore, the foregoing embodiments and advantages aremerely exemplary and are not to be construed as limiting the presentinvention. The present teaching can be readily applied to other types ofapparatuses. The description of the foregoing embodiments is intended tobe illustrative, and not to limit the scope of the claims. Manyalternatives, modifications, and variations will be apparent to thoseskilled in the art. In the claims, means-plus-function clauses areintended to cover the structures described herein as performing therecited function and not only structural equivalents but also equivalentstructures.

What is claimed is:
 1. A method for manufacturing an n-i-p type photovoltaic device including a first unit cell and a second unit cell on which light is incident, the method comprising: forming an n-type semiconductor layer of the first unit cell on a substrate; forming an intrinsic semiconductor layer of the first unit cell on the n-type semiconductor layer, the intrinsic semiconductor layer comprising a mixed phase hydrogenated microcrystalline silicon based material comprising an amorphous silicon based material and crystalline silicon grains; etching a surface of the intrinsic semiconductor layer to partially expose the silicon grains, such that the intrinsic semiconductor layer has a first surface that is textured by the exposed silicon grains and a non-textured opposing second surface; and forming a p-type semiconductor layer of the first unit cell on the intrinsic semiconductor layer, wherein an average crystal volume fraction of the intrinsic semiconductor layer of the first unit cell ranges from 25% to 75%.
 2. The method of claim 1, wherein the substrate is a flexible substrate.
 3. The method of claim 1, wherein the etching the surface of the intrinsic semiconductor layer is performed by a dry-etching process.
 4. The method of claim 3, wherein the dry-etching process is a hydrogen plasma etching process or an argon plasma etching process.
 5. The method of claim 1, wherein the amorphous silicon based material is etched more quickly than the crystalline silicon grain.
 6. The method of claim 1 wherein a passivation layer is formed on the etched surface of the intrinsic semiconductor layer.
 7. The method of claim 6, wherein the passivation layer comprises a hydrogenated microcrystalline silicon based material.
 8. The method of claim 1, further comprising forming an electrode on the second unit cell, wherein an average pitch of a textured surface of the electrode is equal to or more than 50 nm and equal to or less than 500 nm. 